Consistency levels in database systems allow users to trade off correctness for performance by specifying which consistency guarantees are needed from the system for a particular application. The concept of consistency is context-dependent, and there are different types of consistency levels, including sequential consistency, strict consistency, linearizability, causal consistency, and eventual consistency. These consistency levels differ in their requirements on how to order writes and reads, with stronger consistency levels imposing more stringent ordering constraints. Consistency levels can be applied to database transactions by annotating read and write requests with transaction identifiers, which adds additional constraints corresponding to atomicity, isolation, and durability guarantees. Understanding the differences between consistency levels and isolation levels is crucial for designing and implementing distributed systems that meet specific performance and correctness requirements.